In an electronic apparatus including multiple subsystem circuits, each of the subsystem circuits may require a different DC voltage level from another, and hence there is a need for DC power converters. Referring to FIG. 1 showing a schematic diagram of a structure of a DC power converter applied in an electronic apparatus, a system voltage supply V_system is converted via a DC power converter 10 to generate a first voltage V1 having different voltage levels to be provided to subsystem circuits 11 and 12.
There are two common types of DC power converters; one is the so-called low dropout regulator (LDO). FIG. 2(a) shows a schematic diagram of an LDO circuit including a feedback control circuit 20 for an operational amplifier. The feedback control circuit 20 controls a transisitor Q1 based on comparison between a feedback voltage Vf and a reference voltage Vr, so as to generate an output voltage Vo associated with the reference voltage Vr by lowering an input voltage Vi. such as the foregoing V_system, where Vo=Vr in this example. However, a shortcoming of the above is that, a deducted voltage Vi−V0 is consumed by the transistor Q1, rendering inadequate conversion efficiency. In addition, the conversion efficiency gets even more unsatisfactory as a required output current gets larger. It means that when the load of a system gets larger, power lost becomes more.
The other type of DC power converter is a buck converter as shown in FIG. 2(b). Based on a pulse signal φ, a driving circuit 25 generates a pulse width modulator (PWM) control signal having an operation cycle D to control transistors Q2 and Q3 to obtain a voltage conversion of Vo/Vi=D, with a conversion efficiency thereof reaching as high as 90%, which is better than that of the aforesaid LDO. Therefore, buck converters are generally implemented as the foregoing DC power converter 10.
However, in order to save power in many portable electronic apparatuses that use batteries as a primary power supply, the electronic apparatuses generally will automatically switch from an active mode to a quiescent mode when having been idle for a period of time so as to shut down certain unnecessary circuits to achieve power-saving. In addition, having entered the quiescent mode, the buck converter reduces the reference voltage Vr to shorten the operation cycle D of the PWM control signal to further lower the output voltage thereof. For example, an original 1.2V is reduced to 0.7V, expecting that an object of system power-saving may be achieved. Yet, although the output voltage is lowered, switching operations of the transistors Q2 and Q3 in the buck converter are still persistently performing due to an unchanged frequency of the PWM control signal from the driving circuit 25, such that charging and discharging operations do not come to a cease. As a result, power consumed by the buck converter is not effectively reduced, and power-saving effects are rather disappointing as expected in the quiescent mode.
To overcome the aforesaid drawbacks of prior art, it is a primary object of the invention to effectively reduce power wastage.